1. LITHOSPHERE : MAGNETIC FIELD AND GRAVITY FIELD IN EARTH

2. GROUP 14 GROUP MEMBERS : HALIMATU SAADIAH BT NGAH @ ISMAIL
MAYA IZATI AFIDA BT SHAHARIN
MUMTAZ JIHAN BT MOHAMED ABDUL QADIR
FAUZIAH HANIM BT ABD HADI
FATEN HAMAMAH BT MOHAMED
COURSE : SBG 2
LECTURER :

3. INTRODUCTION OF THE LITHOSPHERE
Is the hard shell of the Earth, consisting of the crust and the topmost part of the upper mantle.
It is an average of 100km thick.
It contains the minerals, rocks and soils that humans have used for building materials, metals and agriculture.

4. GRAVITY FIELD
The Earth’s mass exerts an attractive force around the Earth called gravity.
The gravity field of the Earth is an equipotential surface.
We call this equipotential surface the geoid.
For general purposes, the mean sea level is assumed as the surface of the geoid.
There are two ways to measure gravity:
Absolute measurement
Relative measurement

5. A gravity anomaly is the difference between the observed reduced gravity and the normal gravity.
Normal gravity: perpendicular of the Earth’s gravitational force and the centrifugal force caused by the rotation of the Earth, theoretical value.
Observed reduced gravity: gravity measurement reduced to sea level.
The difference is caused by the composition and topography of the Earth, the Earth’s rotation (flattening of the poles), and the dynamic features of the Earth.
gravitational force between two objects determined by their masses and distance between them.

6. differences in density of materials (rocks) in Earth’s interior
produces small differences in local gravity field (anomalies)
can be measured with a gravimeter (attraction of spring to mass)

7. gravity density differences also occur over larger areas: mountains
compensation depth
mass above compensation depth is uniform (isostatically balanced)
--no excess or deficiency in mass; no gravity anomalies--

8. gravity mass above compensation depth is not uniform
-- excess mass of dense mantle below mountain (no crustal root)
compensation depth
generates increased gravity and, thus, a positive gravity anomaly

9. gravity mass above compensation depth is not uniform
-- deficiency of mass below low area (too much crust)
compensation depth
generates decreased gravity and, thus, a negative gravity anomaly

10. MAGNETIC FIELD
> Generated by the convective motion the fluid outer core about the solid inner core.
> The “North geographic pole” corresponds to the “south pole” of the imaginary bar magnetic so that the north needle on a compass points towards the north geographic pole

11. Earth’s magnetic field
surrounds the Earth
• has north and south magnetic poles
• is detected by compasses
• is recorded in rocks and minerals as they cool
• is generated in the Earth’s liquid outer core as
it spins and produces electrical currents
Earth’s field similar
to that for
bar magnet (left)
magnetic N and S
is not the same
as geographic
N and S poles
(bar magnet “tilted”)

12. Earth’s magnetic field
changes through time
migration of magnetic north
change in magnetic north relative
to true north
consequence of rotation of outer core

13. Earth’s magnetic field
reverses over time (north and south poles flip)
--magnetic field lines reverse-- N S
“normal” polarity: north is north and
south is south
“reversed” polarity: north is south and
south is north
after next reversal, compass needle will point south

14. Earth’s magnetic field
how do rocks and minerals acquire magnetism?
rocks and minerals at high temperatures (e.g. molten)
must cool through their Curie temperatures
• above Curie temperature, atoms are random
• below Curie temperature, atoms align in domains
that are independent of each other
• below Curie temperature, atoms align with
magnetic field if one is present (e.g. Earth)

15. Earth’s magnetic field
how do rocks and minerals acquire magnetism?
rocks and minerals that cool through Curie temperature
and stay below that temperature through time
record magnetic field AT THE TIME OF THEIR COOLING
paleomagnetism: study of ancient
magnetic fields in rocks
--reconstruction of past fields--
magnetite common mineral in basalt

16. thick flood basalt sequence in Brazil
Earth’s magnetic field
examine thick sequences of basalts to identify reversals
through time (paleomagnetism)
thick flood basalt sequence in Brazil

17. Earth’s magnetic field
re-construct “normal” and “reversed” for lava sequence

18. Earth’s magnetic field
create time-scale for magnetism
from many observations
see that lengths of
magnetic periods
are not uniform
likely relates to
turbulent flow
of outer core
blue = normal polarity
red = reverse polarity
black = normal polarity
blue = reverse polarity

19. Earth’s magnetic field
what happens during reversals?
geologic evidence
suggests that
reversals occur
quickly
(a few 1000 yrs)
reversed (orange north)
normal (blue north)
computer simulations
indicate that
transitions are
chaotic with
many magnetic poles
in odd places
i.e. not N or S
transitional (chaotic)

20. Earth’s magnetic field
magnetic anomalies occur in local field from magnetic rock
below surface (similar to gravity anomalies)
magnetic material
below “adds”
magnetism
and creates
positive anomaly
magnetic rocks
include
iron ore,
gabbro,
granite

21. Earth’s magnetic field
removal of magnetic material from near surface
causes negative anomaly (example is normal faulting)

22. THANK YOU… ^_^’’